Controllable speed windmill operation of a gas turbine engine through low spool power extraction

ABSTRACT

A gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, a fan section including a fan, a geared architecture, a spool which drives the fan through the geared architecture, and at least one component geared to the spool and being operable to control a speed of the spool during a “windmilling” condition. A method of gas turbine engine operations during a “windmilling” condition is also disclosed.

BACKGROUND

The present disclosure relates to a gas turbine engine, and moreparticularly to engine operations during “windmilling” conditions.

In the highly unlikely event of an in-flight engine shutdown, a gasturbine engine fan has a tendency to spin at above-idle speed as air isforced through the fan due to forward aircraft motion. This unpoweredfan rotation is commonly known as “windmilling”. Even a fan of a shutdown engine on the ground may “windmill” and may require at least somelubrication sufficient to sustain gears, bearings, seals, and otherlubricated components.

SUMMARY

A gas turbine engine according to an exemplary aspect of the presentdisclosure includes a spool driven by a geared architecture at least onecomponent geared to the spool to control a speed of the spool during a“windmilling” condition.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features will become apparent to those skilled in the art fromthe following detailed description of the disclosed non-limitingembodiment. The drawings that accompany the detailed description can bebriefly described as follows:

FIG. 1 is a schematic cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic cross-sectional view of the gas turbine enginewithin a nacelle assembly;

FIG. 3 is an enlarged schematic view of a thermal system with anintegrated Thermal Management System (TMS) and Environmental ControlSystem (ECS) for the gas turbine engine;

FIG. 4 is a schematic view of an integrated air-oil cooler (AOC)/air-airprecooler;

FIG. 5 is a perspective view of a duct arrangement with a scoop and abypass flow duct within a core nacelle of the gas turbine engine;

FIG. 6 is a schematic view of the duct arrangement in bypass flowposition;

FIG. 7 is a schematic view of the duct arrangement in a bleed flowposition;

FIG. 8 is a schematic view of the duct arrangement in an intermediateposition;

FIG. 9 is a schematic view of one disclosed non-limiting embodiment of aconstant speed transmission which drives an ECS pump and a TMS pump in aserial arrangement;

FIG. 10 is a schematic view of another disclosed non-limiting embodimentof a constant speed transmission which drives an ECS pump and a TMS pumpin a parallel arrangement;

FIG. 11 is a schematic view of an accessory and thermal system driven bythe gas turbine engine; and

FIG. 12 is a schematic view of another disclosed non-limiting embodimentof a constant speed transmission which drives an IDG, a windmill pump,an ECS pump and a TMS pump.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmentor section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flowpath whilethe compressor section 24 drives air along a core flowpath forcompression and communication into the combustor section 26 thenexpansion through the turbine section 28. Although depicted as aturbofan gas turbine engine in the disclosed non-limiting embodiment, itshould be understood that the concepts described herein are not limitedto use with turbofans as the teachings may be applied to other types ofturbine engines, such as three-spool architectures.

The engine 20 generally includes a low spool 30 and a high spool 32mounted for rotation about an engine central longitudinal axis Arelative to an engine static structure 36 via several bearing systems38. It should be understood that various bearing systems 38 at variouslocations may alternatively or additionally be provided.

The low spool 30 generally includes an inner shaft 40 that interconnectsa fan 42, a low pressure compressor 44 and a low pressure turbine 46.The inner shaft 40 may be connected to the fan 42 directly or through ageared architecture 48 to drive the fan 42 at a lower speed than the lowspool 30 which in one disclosed non-limiting embodiment includes a gearreduction ratio of greater than 2.4:1. The high spool 32 includes anouter shaft 50 that interconnects a high pressure compressor 52 and highpressure turbine 54. A combustor 56 is arranged between the highpressure compressor 52 and the high pressure turbine 54. The inner shaft40 and the outer shaft 50 are concentric and rotate about the enginecentral longitudinal axis A which is collinear with their longitudinalaxes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The turbines 54, 46 rotationally drive therespective low spool 30 and high spool 32 in response to the expansion.

With reference to FIG. 2, the gas turbine engine 20 is mounted to anengine pylon structure 60 within an engine nacelle assembly 62 as istypical of an aircraft designed for subsonic operation. The nacelleassembly 62 generally includes a core nacelle 64 and a fan nacelle 66.It should be appreciated that the core nacelle 64 and the fan nacelle 66may be of various configuration and may be at least partially integratedadjacent to, for example, an upper bi-fi and a lower bi-fi to definewhat are often referred to as D-doors.

The fan nacelle 66 is at least partially supported relative to the corenacelle 64 by Fan Exit Guide Vanes (FEGVs) 68 which extend between acore case 70 and a fan case 72. The core case 70 and the fan case 72 arestructural members that support the respective fan nacelle 66 and corenacelle 64 which define outer aerodynamic surfaces around the core case70 and the fan case 72. The core case 70 is often referred to as theengine backbone and supports the rotational componentry therein. Itshould be understood that although a particular component arrangement isdisclosed in the illustrated embodiment, various pylon structures,nacelle assemblies and engine case structures will benefit herefrom.

An annular bypass flow path 74 is defined between the fan nacelle 66 andthe core nacelle 64. The engine 20 generates a high bypass flowarrangement with a bypass ratio in which approximately eighty percent ofthe airflow which enters the fan nacelle 66 becomes bypass flow. In thedisclosed non-limiting embodiment, the bypass flow communicates throughthe generally annular bypass flow path 74 and may be discharged from theengine 10 through a variable area fan nozzle (VAFN) 76 which defines avariable exit area for the bypass flow.

As the fan blades within the fan section 22 are efficiently designed ata particular fixed stagger angle for an efficient cruise condition, theVAFN 76 is operated to effectively vary the fan nozzle exit area toadjust fan bypass air flow such that the angle of attack or incidence onthe fan blades is maintained close to the design incidence for efficientengine operation at other flight conditions, such as landing and takeoffto thus provide optimized engine operation over a range of flightconditions with respect to performance and other operational parameterssuch as noise levels.

With reference to FIG. 3, the engine 20 includes a thermal system 80(illustrated schematically) powered by the low spool 30. The thermalsystem 80 integrates a Thermal Management System (TMS) 82 and anEnvironmental Control System (ECS) 84 powered by the low spool 30.

The TMS 82 generally includes a TMS pump 86 such as an axial fan and anair-oil cooler (AOC) 88 which is in fluid communication with an enginelubrication system to cool engine oil. The ECS 84 generally includes anECS pump 90 such as an impeller within a scroll discharge 91 and anair-air precooler 92 which operates to cool air for use in the aircraftcabin. The flow passes through the air-oil cooler (AOC) 88 to coolengine oil then through the air-air precooler 92 to cool the relativelyhot ECS air.

In one disclosed, non-limiting embodiment, the coolers 88, 92 areintegrated into one unit 93 to reduce system weight, size, andcomplexity. It should be appreciated that two or more coolers may be sointegrated such that a cooling air flow passes through the air-oilcooler (AOC) 88 and then directly into air-air precooler 92. Arrangementof the air-oil cooler (AOC) 88 and the air-air precooler 92 in directseries as a single unit within a common housing 89 (FIG. 4) provides fora reduction in the overall packaging volume with reduced weight due inpart to elimination of separate inlet and exit duct geometries. Thesandwich structure also eliminates transition duct length associatedwith separate coolers and connecting flanges, as well as locates the finmedia closer together to further reduce package volume.

For volume-challenged engine architectures, the decreased packagingvolume trades favorably against a relatively small weight increase asthe air-air precooler 92 may be sized somewhat larger than otherwiserequired to match a rectilinear shape and flow path geometry of theair-oil cooler (AOC) 88. That is, the geometry of the integral unit mayresult in one of the air-air precooler 92 or the air-oil cooler (AOC) 88to be physically oversized. Such an “oversized” relationshipadvantageous provides overly efficient operation and may somewhatincrease weight—yet still less than separate coolers—as a tradeoff forelimination of separate inlet and exit duct geometries of separatecoolers.

Fan bypass air from a scoop 94 within the bypass flow path 74 isselectively communicated to the TMS pump 86 and the ECS pump 90 througha bypass flow duct 96 within the core nacelle 64 (FIG. 2). The scoop 94and bypass flow duct 96 in the disclosed non-limiting embodiment may bemounted to the core case 70 (FIG. 5) independent of the core nacelle 64such that the core nacelle 64 is readily opened and closed with respectto the core case 70 without the heretofore necessity of a seal structurewhich may be relatively heavy in weight. That is, the scoop 94 and thebypass flow duct 96 are independent of the core nacelle 64 sectioncommonly referred to as D-doors.

Relatively hot bleed air sourced from the low pressure compressor 44 isalso selectively communicated to the TMS pump 86 as well as the ECS pump90 through a compressor flow duct 98. The compressor flow duct 98communicates bleed air from the low pressure compressor 44. It should beappreciated that various duct and valve arrangements as may be utilizedto tap the core case 70 to communicate bleed air from a multiple ofcircumferential locations around the low pressure compressor 44 forcommunication into the compressor flow duct 98.

The bypass flow duct 96 meets with the compressor flow duct 98 at anintersection 100. A valve 102 is located within the intersection 100 toselectively communicate either fan bypass flow from the bypass flow duct96 (FIG. 6) or bleed flow from the compressor flow duct 98 (FIG. 7) tothe TMS pump 86 and the ECS pump 90. That is, the valve 102 is movablebetween a first position (FIG. 6) and a second position (FIG. 7) toselectively communicate either fan bypass flow or bleed flow.

The valve 102 may be operated by an actuator 104 in response to acontroller 106, such as a FADEC, to selectively communicate, forexample, compressor bleed flow from the compressor flow duct 98 (FIG. 7)during an idle condition when fan bypass flow from the bypass flow duct96 may not provide sufficient mass flow. It should be understood thatvarious other conditions may be utilized to control the valve 102 whichmay alternatively or additionally be operated in a variable manner toprovide a combined flow of fan bypass flow from the bypass flow duct 96and bleed flow from the compressor flow duct 98 (FIG. 8). In otherwords, the valve 102 may be infinitely variable between the firstposition (FIG. 6) and the second position (FIG. 7) to provide a desiredpercentage of each.

In one disclosed, non-limiting embodiment, the ECS pump 90 may be acentrifugal pump and the TMS pump 86 may be an axial pump. The TMS pump86 generates, for example, an approximately 1.1:1-1.8:1, and preferably1.4:1, pressure ratio from the relatively low pressure ratio fan bypassflow which is sufficient to provide the relatively coldest airflow intothe AOC 88, which may be approximately 200 degrees F. The relatively lowpressure ratio fan bypass flow from the bypass flow path 74 is alsoprovided to the ECS pump 90 to elevate the pressure thereof to, forexample, an approximately 2:1-6:1, and preferably 4:1, pressure ratio atground idle condition. The pressure increase provided by the ECS pump 90also inherently increases temperature of the approximately 200 degreesF. fan bypass flow to less than 600 degrees F. for communication intothe air-air precooler (PC) 92.

The downstream flow from the air-oil cooler (AOC) 88, which may beapproximately 300 degrees F., is communicated into the air-air precooler92. Discharge from the air-air precooler 92, which may be less thanapproximately 600 degrees F., is then ejected into the annular bypassflow path 74 to provide thrust recovery. That is, the relatively lowertemperature air flow downstream of the ECS pump 90, which is typicallyless than approximately 600 degrees F., is passed through the air-airprecooler 92 and is cooled to approximately 400 degrees F. for use asaircraft air system ECS air while the relatively higher temperature airdischarged from air-air precooler 92, which may be less thanapproximately 600 degrees F., is ejected into the annular bypass flowpath 74 to provide thrust recovery. An efficient and compact thermalsystem 80 is thereby provided.

The downstream flow from the air-air precooler 92 may also be utilizedto provide pressurized cooling air for a hot bearing compartment for oneor more of the bearing systems 38. Such components are typically towardan aft section of the engine 20 such as the #4 or #4/5 bearingcompartments (illustrated schematically). The fan bypass flow is pumpedto sufficient pressure (typically approximately 50 psi) and passedthrough the aircraft precooler 92 to reduce temperature sufficiently(typically to less than 450 F) to be used directly as the bearingcompartment cooling air. The precooler 92 thereby provides sufficientlylow temperature air, instead of a dedicated buffer cooler, which maysuffer from low inlet driving pressure at off-design conditions.

The TMS pump 86 and the ECS pump 90 are driven through a constant speedtransmission 110. The constant speed transmission 110 is driven by atowershaft 124 geared to the low spool 30. The speed of the towershaft124 varies linearly with the speed of the low spool 30 which may operateat speed excursions of up to 80% between idle to max take-offconditions. The constant speed transmission 110 maintains constantoutput speed despite speed excursions of the low spool 30. That is, theconstant speed transmission 110 provides, for example, a 5:1continuously variable gear ratio capability which automatically selectsthe most optimum gear ratio to maintain the constant output speed inresponse to the low spool 30 speed excursions.

With reference to FIG. 9, in one disclosed non-limiting embodiment, theTMS pump 86 and the ECS pump 90 are driven through the constant speedtransmission 110 with a single axial drive shaft 112. That is, the TMSpump 86 and the ECS pump 90 are driven at the same rotational speedalong a common axis X by shaft 112.

With reference to FIG. 10, in another disclosed non-limiting embodiment,the TMS pump 86 and the ECS pump 90 are driven through the constantspeed transmission 110 through separate drive shafts 114, 116respectively. That is, the drive shafts 114, 116 are parallel and rotateabout their own respective axis X1, X2.

The parallel architecture facilitates direct drive of the drive shaft114 by the constant speed transmission 110 while drive shaft 116 isdriven by drive shaft 114 through a gearbox 118 or vice-versa. Gearbox118 may be a direct, step-down or step-up gearbox such that shaft 116 isdriven at the same rotational speed as shaft 114 or at a respectivespeed ratio with respect to shaft 114. In other words, the constantspeed transmission 110 provides a constant output speed for shafts 114,116 irrespective of low spool 30 speed excursions, and gearbox 118provides a desired constant speed ratio between shafts 114 and 116.

Utilization of the constant-speed TMS pump 86 to drive air-oil cooler(AOC) 88 air flow increases the available pressure ratio for oilcooling. Power extraction from the relatively high-inertia low spool 30also affects engine performance less adversely than does powerextraction of a similar magnitude from the high spool 32.

With reference to FIG. 11, the high spool 32 may still be utilized todrive a relatively conventional accessory gearbox 120 to power amultiple of accessory components such as, for example, a deoiler (D), ahydraulic pump (HP), a lube pump (LP), a permanent magnet alternator(PMA), a fuel pump module (FMP), and other accessory components 121 thatmay alternatively or additionally be provided.

A high towershaft 122 is geared to the high spool 32 to drive theaccessory gearbox 120 at a speed that varies linearly with the speed ofthe high spool 32. The high spool 32 operates at speed excursions lessthan the low spool 30 and typically of only up to 30% between idle tomax take-off conditions. Power extraction from the relativelylow-inertia high spool 32 for operation of low demand accessorycomponents minimally affects engine performance. That is, the thermalsystem 80 includes relatively high demand, high power systems which aremore constantly operated to provide a desired speed/mass flow ascompared to the accessory components driven by the high spool 32.

Utilization of the low spool 30 driven thermal system 80 increasesoperating range and decrease packaging volume. Integration of the,air-air precooler 92 into the common cooling/exit stream of the Air-OilCooler (AOC) 88 provides thrust recovery of the air-air precooler 92discharge as compared to legacy configurations that dump precoolerdischarge flow overboard outside the fan bypass duct typically throughthe pylon fairing “thumbnail” or similar aircraft surface exposed tofree stream air which negates thrust recovery benefits.

The dedicated ECS subsystem relieves the high spool 32 frominefficiencies and distortion due to bleeds at design and off-designpoints. ECS mass flow is approximately 1 lb per second, and efficiencygains from not bleeding this air from the high pressure compressor areabout +2% HPC efficiency if power is instead extracted from the lowspool, with reduced distortion due to lack of environmental controlsystem bleeds. Exhaust gas temperature (EGT) at idle may also decreaseby more than 230 degrees F. Overall system weight also decreases due tothe reduced ducting. Accordingly, valuable externals packaging space isfacilitated by the reduction and integration of the TMS and ECS.Further, mechanical complexity is reduced to increase reliability aswell as reduce cost and maintenance requirements.

With reference to FIG. 12, another disclosed non-limiting embodimentconnects an integrated drive generator (IDG) 130 and a windmill pump 132to the constant speed transmission 110. It should be appreciated thatadditional or alternative components and systems 111 (shownschematically in FIGS. 11 and 12) may be driven by the constant speedtransmission 110. As the constant speed transmission 110 is driven bythe low spool 30, the constant speed transmission 110 is driven evenunder “windmilling” conditions.

Under on-power operation, the integrated drive generator (IDG) 130, theTMS pump 86 and the ECS pump 90 draw, in one example, between 90 kVA and200 kVA (96-214 hp).

In a “windmilling” condition, the TMS pump 86, the ECS pump 90, theintegrated drive generator (IDG) 130 and the windmill pump 132 stilloperate and such operation may be utilized to control and reduce thespeed of the fan 42 to a desired “windmilling” speed. Such speedreduction is achieved even in the unlikely presence of an imbalancewhile “windmilling” after fan blade liberation.

Applicant has determined through simulation that in one example gearedarchitecture gas turbine engine, upwards of 200 hp of power extractionis available at acceptable fan 42 speeds during “windmilling”conditions. The various components driven by the constant speedtransmission 110 and gear ratios provide multiple variables to achieve adesired “windmilling” speed of the fan 42 as well as the desired powergeneration of the integrated drive generator (IDG) 130, provide desiredoperational conditions of the TMS pump 86, the ECS pump 90, and thewindmill pump 132. That is, the low spool 30 driven systems draw arequired amount of power from the low spool 32 to operate as a brake toslow the low spool 32 to acceptable levels.

In one disclosed, non-limiting embodiment, the windmill pump 132 isalternatively or additionally “piggybacked” to the TMS pump 86 and/orthe ECS pump 90. Alternately, the windmill pump 132 is independentlydriven such as in the parallel arrangement described above (FIG. 10).

The windmill pump 132 pumps a lubricant to, for example, the gearedarchitecture 48 to assure proper lubrication for an in-flight and ground“windmilling” condition. The windmill pump 132 further facilitateslubrication delivery for both the relatively higher in-flight“windmilling” condition as well as the relatively lower ground“windmilling” speed due to the geared relationship with the low spool32. It should be understood that the windmill pump 132 may beselectively actuated or operated at constant speed throughout themission cycle as opposed to varying with a speed of the gearedarchitecture 48. Losses at an Aerodynamic Design Point (ADP; Cruiseflight) and non-windmill conditions are reduced. Windmill pump size andweight are decreased compared to a core engine embedded version. Inaddition, the windmill pump 132 is remote from the engine core tofacilitate increased maintenance accessibility as compared toconventional windmill pumps which are otherwise “buried” deep within theengine.

As the windmill pump 132 is driven by the low spool 30 through theconstant speed transmission 110, the individual accessories are therebyinsulated from the speed excursions of the low spool 30. For example,takeoff condition at 100% speed and an idle condition at 20% speed. So,the accessory components thereby experience a gear ratio advantage of5:1 during normal operation. The windmill pump 132 is currently designedto work at low spool 30 speeds of just above “0” rpm to idle, and thencovers the entire 80% speed excursion of the low spool 30. Since thewindmill pump 132 is also driven through the constant speed transmission110 the windmill pump 132 need only handle the speed excursion between“0” rpm and the idle speed condition. The remainder—idle to takeoffcondition—is accounted for by the constant speed transmission 110. Thisfacilities operation of a lubrication system with an optimized pump thathandles narrow speed excursions, while being line replaceable.

The low spool 30 driven integrated drive generator (IDG) 130 furtherfacilitates integral power generation under “windmilling” conditionssuch that a Ram Air Turbine (RAT) emergency power generation system mayno longer be required. Removal of the RAT reduces overall aircraftweight and obviates RAT deployment structures and packaging.

It should be understood that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould also be understood that although a particular componentarrangement is disclosed in the illustrated embodiment, otherarrangements will benefit herefrom.

Although particular step sequences are shown, described, and claimed, itshould be understood that steps may be performed in any order, separatedor combined unless otherwise indicated and will still benefit from thepresent disclosure.

The foregoing description is exemplary rather than defined by thelimitations within. Various non-limiting embodiments are disclosedherein, however, one of ordinary skill in the art would recognize thatvarious modifications and variations in light of the above teachingswill fall within the scope of the appended claims. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced other than as specifically described. For that reasonthe appended claims should be studied to determine true scope andcontent.

What is claimed is:
 1. A gas turbine engine comprising: a fan sectionincluding a fan; a geared architecture; a spool which drives said fanthrough said geared architecture; at least one component geared to saidspool and being operable to control a speed of said spool during a“windmilling” condition; and wherein said at least one component isoperable to reduce a speed of the fan to a predetermined threshold inresponse to the “windmilling” condition.
 2. The gas turbine engine asrecited in claim 1, wherein said at least one component includes awindmill pump driven through a constant speed transmission.
 3. The gasturbine engine as recited in claim 1, wherein said at least onecomponent includes an axial pump driven through a constant speedtransmission.
 4. The gas turbine engine as recited in claim 1, whereinsaid at least one component includes a centrifugal pump driven through aconstant speed transmission.
 5. The gas turbine engine as recited inclaim 1, wherein said at least one component includes an IntegratedDrive Generator (IDG) driven through a constant speed transmission. 6.The gas turbine engine as recited in claim 1, wherein said spool is alow spool coupled to a low pressure turbine.
 7. The gas turbine engineas recited in claim 1, wherein said at least one component includes awindmill pump fluidly coupling said geared architecture and a lubricantsupply, and said windmill pump is mechanically coupled to said spoolthrough a constant speed transmission such that rotation of said fandrives said windmill pump through said constant speed transmission. 8.The gas turbine engine as recited in claim 2, wherein said windmill pumpis configured to provide lubricant to said geared architecture.
 9. Thegas turbine engine as recited in claim 8, wherein said windmill pump isconfigured to provide lubricant to said geared architecture during boththe “windmilling” condition and an on-power condition.
 10. A gas turbineengine comprising: a fan section including a fan; a geared architecture;a spool which drives said fan through said geared architecture; aconstant speed transmission driven by said spool; and at least onecomponent driven by said constant speed transmission and being operableto control a speed of said spool during a “windmilling” condition. 11.The gas turbine engine as recited in claim 10, wherein said at least onecomponent includes a windmill pump.
 12. The gas turbine engine asrecited in claim 11, wherein said at least one component includes anaxial pump.
 13. The gas turbine engine as recited in claim 11, whereinsaid at least one component includes a centrifugal pump.
 14. The gasturbine engine as recited in claim 11, wherein said at least onecomponent includes an Integrated Drive Generator (IDG).
 15. The gasturbine engine as recited in claim 10, wherein said spool is a lowspool.
 16. The gas turbine engine as recited in claim 15, comprising ahigh spool interconnecting a high pressure compressor to a high pressureturbine, and an accessory gearbox driven by said high spool.
 17. The gasturbine engine as recited in claim 16, wherein said high spool drivessaid accessory gearbox at a first speed that varies linearly with asecond speed of said high spool.
 18. The gas turbine engine as recitedin claim 16, comprising at least one auxiliary device driven by saidaccessory gearbox, wherein a combined first power demand of each of saidat least one auxiliary device is less than a combined second powerdemand of each of said at least one component.
 19. The gas turbineengine as recited in claim 15, wherein said constant speed transmissiondefines a 5:1 continuously variable gear ratio.
 20. A method of gasturbine engine operations during a “windmilling” condition comprising:controlling a speed of a spool during the “windmilling” condition withat least one component driven by a constant speed transmission driven bythe spool.
 21. The method as recited in claim 20, wherein the spooldrives a fan.
 22. The method as recited in claim 20, wherein the spooldrives a fan through a geared architecture.
 23. The method as recited inclaim 22, wherein the step of controlling includes reducing a speed ofsaid fan to a predetermined threshold.